hello and welcome to the review of chapter 74 of guidance medical physiology textbook in this chapter we're going all over the body temperature regulation and what happens when our body temperature regulation system fails if you enjoy the chapter please don't forget to give it a like and subscribe to the channel to help the small channel grow so to begin with we have two types of temperature within our body we've got our skin temperature which is obviously just the temperature of our skin and then core temperature which is deeper within our internal organs so our core temperature is obviously what we want to maintain our skin temperature actually fluctuates quite wildly and is able to be altered in order to alter our cool temperature and maintain our core temperature pretty normal now our normal core temperature there is a range anywhere between 97 and 99.5 degrees fahrenheit and then for our non-american listeners out there 36 to 37.5 celsius obviously this body temperature is going to increase with exercise decrease when we're in cold environments ultimately we want to maintain a very normal temperature within this range so then our chemical systems within our body can go on as per normal our brain can continue the function and we don't denature our proteins with excessive heat or we don't freeze our cells when it gets too cold so as we mentioned in our last chapter we do have the ability to increase our heat production through metabolism now there are several ways we can do that we've got our basal rate of metabolism so that's just all our chemical reactions within our body we have muscle activity that obviously increases heat we can increase metabolism with some hormones so thyroxine growth hormone testosterone epinephrine and norepinephrine can also increase metabolism and then also just the chemical activity within the cells will increase as the cell temperature increases lastly here we have the thermogenic effect of food so remember there is heat production just by digesting absorbing and storing food heat is mainly lost through the skin so the rate of heat loss is determined by how rapidly that heat can go to the skin and then how rapidly that heat is going to go from the skin to the environment we can increase heat going to the skin by increasing blood supply to the skin and then we can increase the heat loss from our skin by various methods we'll get to later on in this chapter now we do have some insulators in our body mainly fat since fats poorly conducts heat so if we have a lot of fat in our skin that's going to take a longer amount of time to transfer that heat to the skin and then it's also going to store a lot more heat and not allow it to be lost as quickly now when it comes to the actual skin histology picture here we have the epidermis which is the outer layer the dermis and in our subcutaneous tissue which contains all the fat now the blood vessel supply to the skin is very important here it's slightly different where we actually have this venous plexus here this venous plexus right in the middle is supplied by some arteriovenous anestmoses and there's also the draining area for the veins where once blood has supplied the outer areas of the skin and it starts to be drained it will end up in the venous plexus as well so we're able to increase the blood within the venous plexus which allows more heat to be transferred to the environment by increasing the blood flow via the arteriovenous anastomosis artery so if we have vasodilation and we have increased blood flow to this venous plexus we get increased heat loss if we have vasoconstriction less blood flow here we get conservation of our heat as you can see in this figure 74 3 here vasoconstriction reduces our heat conductive through the skin as the environmental temperature rises we start to vasodilate our arteries to our skin to allow increased heat conduction because we obviously don't want to get too hot so we want to get rid of the extra heat so there are actually three main mechanisms for us to lose heat from our body and that's shown in this figure 74 4 here so the first one is just radiation of heat waves everything that is not at absolute zero temperature has these radiation heat waves via infrared rays we can't see these rays but they're being emanated from all surfaces so there's radiation heat waves even coming from the walls and all the objects in your room as well as coming from your skin as well so if you are warmer than your environment then you will be emitting more of these radiation heat waves then you will be absorbing from the environment so if your body temperature is warmer than your environment you'll be losing more heat via radiation whereas if you are cooler than your environment you will be gaining heat via radiation so that's mechanism number one mechanism number two is conduction slash convection so conduction just literally means all of the molecules in your skin that have a natural little vibration to them they are going to vibrate any molecules that are immediately next to them so that's going to vibrate any solid molecules that you're sitting on or touching so that's automatically going to transfer some energy to any solid objects which is a relatively small amount of heat transfer but it's also going to vibrate and transfer energy over to ear molecules that are next to you so that is conduction conduction is just transferring that vibration energy from one molecule to the next with whatever's in contact with your skin now obviously if those air molecules just stay right next to your skin you're going to transfer energy make them vibrate and then they're not going to take any more energy because they're just vibrating next to your skin if the air molecule gets removed and gets replaced with another air molecule then that's going to take some energy away from your skin as it starts to vibrate so that's what convection is convection is removing all of those air molecules that you have just transferred your heat to and replacing it with new air molecules to receive more heat so convection just allows greater conduction of air so if you are in a windy environment and your skin is exposed you're going to emit more heat because you're transferring more heat via just passive vibration of your molecules to the nearby ear molecules so that's mechanism number two is conduction convection and then mechanism number three is evaporation so evaporation is literally just the process of evaporating a water molecule obviously if we're sweating we're going to have greater evaporation because you're releasing more water onto your skin and you're going to have greater evaporation so that's what we're going to do to transfer heat away from our body when we are hot but if we're not hot we are still actually losing water molecules via evaporation even without sweating you have a small amount of water that automatically evaporates from your skin regardless of whether you're sweating or not and then also from your lungs as well because remember your lungs have to stay humidified and as you're breathing in that air in and out you're going to result in the loss of water via evaporation so that's your third mechanism of heat transfer is via the evaporation of water and the evaporation of water that's not involving sweating is called insensible losses so we have a normal basal amount of insensible losses which increases when you're in dry environments decreases when you're in a humid environment because in dry environments you're going to lose more water to evaporation so those are your three mechanisms radiation conduction convection and evaporation now clearly we can change some of these heat losses by actually wrapping ourselves in clothes if we wrap ourselves in clothes we actually trap a layer of air around our skin so then that ear cannot leave and be replaced with a new air molecule so we actually keep our heat within our body and we also slightly reduce the amount of radiation that leaves our body as well now an important concept there is that if you actually get your clothes wet then you lose the air pocket and water actually has a higher conductivity than air so you actually lose your heat as your body vibrates those water molecules which vibrates all the water molecules next to it and increases the heat's loss because if you are in a water environment water is able to suck away heat from you because it's able to just transfer all that vibrating molecules and all the extra energy away from the body so water has a higher conductivity meaning that if you have wet clothes on you are going to increase the heat loss whereas if you have dry clothes on you're trapping a layer of air that can't go anywhere and the lower conductivity of air means that you're able to reduce your heat loss now sweating obviously is going to increase our evaporation as we've talked about sweating is innovated by our sympathetic nervous system because when we are exercising and we increase our body temperature we need to be able to sweat to actually maintain a normal core temperature by increasing evaporation in our skin now as with any glandular secretion we have the primary secretion within the sweat gland in this coiled portion which is basically just plasma without plasma proteins and then as that gets released there is absorption of sodium and chloride trying to maintain our electrolyte balance so then we release a substance mainly full of chloride urea and lactic acid and hopefully some water to be evaporated now if we only stimulate sweating just a small amount then we're only going to produce a small amount of sweat and then a lot of sodium and chloride almost all of it's going to be reabsorbed and with it a lot of water so we're just going to basically get rid of a little bit of water and mainly just wastes whereas when we have a strong stimulation then we're going to dramatically increase the production of sweat it's not going to have enough time to absorb sodium and chloride as it goes through the tubular duct and then you're going to release a lot more sodium chloride and a lot more water as you get acclimatized you're able to produce more sweat and absorb more sodium chloride so then you're able to maintain your sodium chloride balance and yet still be able to release sweat without dramatically affecting your electrolyte balance and this has helped with aldosterone because aldosterone increases your plasma volume and increases the absorption of your sodium and chloride remember we've talked about aldosterone back in the renal chapter how that influences the distal tubule that's able to increase the plasma volume so we can lose more fluid but then also reabsorb more sodium and chloride as well in animals that cannot sweat so a lot of animals with fur obviously their skin is actually not covered with any sweat glands they have a panting mechanism which is just very short shallow breaths where you're not actually altering the ventilation rate so the alveolar stalks getting the same amount of oxygen and releasing the same amount of carbon dioxide you're just moving a large quantity of air through your respiratory passages allowing more evaporation more conduction convection so next up is talking about the regulation of body temperature and as you can see as atmospheric temperature alters our body temperature is maintained at a pretty steady rate at least for the majority of atmospheric temperatures as we get to very cold temperatures alpha systems start to fail and as we get to very hot temperatures our system starts to fail but otherwise we're able to regulate it at a pretty normal temperature within the middle here and so we'll go through those various mechanisms very shortly but first we have to talk about the hypothalamic pre-optic area that is our thermostat of our body so the hypothalamic pre-optic area is able to sense whenever it is too hot or if it's too cold to then correspondingly set off a series of responses to then either increase or decrease our body temperature so our pre-optic area if it is heated it will automatically vasodilate all our arteries in our skin cause profuse sweating and then also reduce our metabolism to reduce our heat production in our body whereas if the body is too cold it's going to stimulate vasoconstriction in the body pilo erection which is getting the hair molecules on our skin to stand up straight so then that layer of air around our skin gets trapped so reducing conduction convection and then also increase our heat production by increasing our metabolism and it does that through sympathetic stimulation and thyroxine stimulation so that is how the pre-optic area works but we also have some other sensory areas as well in our skin and also in our spinal cord and abdominal viscera as well these receptors do have some warm receptors but it's mainly for cold reception mainly for preventing hypothermia so it's able to sense cold and then stimulates local shivering inhibits sweating and also promotes local vasoconstriction as well so i mean local where if it's just one hand or one foot that's in an ice bucket then you will then cause vasoconstriction inhibit sweating in that leg and then also promote shivering in just that leg as well so it's got this local feedback that actually gets promoted and intensified by the pre-optic area in the hypothalamus now there is actually a shivering center in the hypothalamus as well which if that's gets stimulated how it works is that it actually just increases the tone of the skeletal muscle throughout the body and when that tone ranges raises to a certain critical level it results in shivering because of a feedback oscillation with the stretch reflex mechanism of your muscles so you've increased that tone of that muscle enough to then stimulates the stretch reflex so then that results in an inhibition so then it relaxes but then your tone is naturally increased so it automatically contracts and then it gets sensed by the local reflex to then relax again so then it's constantly constricting and relaxing resulting in the shivering mechanism and then that increases heat production now sympathetic stimulation also causes heat production through non-shivering thermogenesis and that's just by increasing the metabolism of our body but then also by uncoupling oxidative phosphorylation what uncoupling of oxidative phosphorylation means is that you turn food stuffs into energy for heat rather than atp so it's just getting broken down just for the production of heat rather than actually forming a store of energy and that obviously is to increase our body temperature now that is pretty proportional to the amount of brown fat which is in much higher quantities than younger people or in infants and that's because brown fat has a lot of mitochondria and some proteins which is able to turn those foodstuffs into purely heat when it comes to thyroxine if the hypothalamic pre-optic area is cooled down so then we need to increase our heat production it releases thyrotropin releasing hormone from the hypothalamus goes down to our anterior pituitary gland which then stimulates thyroid stimulating hormone that then stimulates our thyroid glands to increase the production of thyroxine thyroxine then activates these uncoupling proteins that we've just mentioned and it increases metabolism around the body so animals in extreme cold for several weeks increase the amount of thyroid hormone they can produce but thyroid hormone in general is able to increase our heat production and there will be more information about this in the endocrine section now an important concept here is the set point for our temperature control so the set point is 37.1 degrees celsius for non-americans and in 98.8 fahrenheit for americans so what that means is that if the body temperature reduces below this level we will activate heat production and reduce heat loss whereas if we go higher than 37.1 celsius 98.8 fahrenheit if we go higher than that then we're going to increase heat loss reduce heat production and then try to get back to the set point of 37.1 now this is also dependent on our skin temperature as well where our skin temperature can alter our set points because if our skin temperature has been altered by one of these mechanisms we don't want to overdo the mechanism and then actually end up in the other scenario so this is our negative feedback system and what i mean by that is that for instance in this figure 74 8 here in this figure 74 9 if we stick to 74 8 for the meantime this is if we have a raising body temperature so if our skin temperature is normal so 33 to 39 degrees celsius is a greater range here because it obviously is able to change with the environment then our set point is that 37.1 so if we increase our body temperature above 37.1 then that will result in sweating so evaporative losses will increase but if our skin temperature is actually quite low so our skin temperature is 29 degrees celsius then sweating will not occur until our actual core body temperature is up near the 37.4 degrees celsius range because if we have a high body temperature but low skin temperature that must mean that we are somewhat trying to control our body temperature and if we actually increased our sweating and we were profusely sweating with a skin temperature of 29 degrees then we may actually result in the opposite problem lose too much heat and then suddenly our core body temperature plummets so our skin temperature will alter the set point and that happens at the same time when we're reducing our body temperature too when it comes to shivering so our shivering set point is said that you know around 37.1 but if our skin temperature is actually quite warm then we're not going to shiver until we're way down at you know 36.5 roughly because we must be in a warm environment and we don't need to shiver to increase our heat production where our core body temperature will probably increase just from the room environment if we are to shiver when our skin temperature is 31 degrees and our body temperature has only just gone below that set point we're probably going to increase our heat production over to what we need and actually become hyperthermic instead so this feedback system is very very good at controlling our body temperature our feedback gain if you remember back to our very first chapter is very good at around about 27 so it's a high gain biological control of body temperature because we need to maintain normal body temperature for our normal systems to occur or else we'll end up with organ failure cns depression and if we're too cold we're going to result in frostbite and gangrene as we'll get to now we've talked about the unconscious control of our body temperature but obviously our behavior also gets altered whenever we get too hot or too cold so our behavior will adapt to increase or decrease our temperature based on how we're feeling whether we're too hot or too cold so what happens when our body temperature regulation fails so that means that we are either too hot or too cold now obviously if we have a high body temperature that means that we may have a fever we may be hypothermic now if we talk about the fever first a fever or pyrexia means that there is something within our body that has actually changed our set point so now that set point is no longer 37.1 but maybe 39 degrees so now all our body temperature regulation is trying to increase our body temperature to maintain it at 39 degrees so a fever or pyrexia means that our set point for our body temperature has increased so the set point has increased all our regulatory systems have now altered to now keep our body temperature at the higher level so we're going to feel cold at normal temperatures which is the chill portion of having a fever you know when you feel like you have the chills but you're actually normal temperature it's because your body's trying to shiver and get yourself up to that new set point and then once a process that was actually telling your body that your new set point is high is removed and your set point goes back to normal suddenly you feel quite hot and you sweat and you're trying to reduce your temperature so what can actually increase the set point of your body temperature what causes a fever and as you would know from personal experience most likely that's various infections viral infections etc so anything that contains a pyrogen will increase your set points and we have both exogenous meaning not in our body pyrogens and that includes you know endotoxins from bacteria those lipopolysaccharides etc these proteins you break down products from bacteria or viruses or even from cancers that can actually increase your set points and then we have endogenous factors as well which come from our immune system so for instance interleukin-1 is a cytokine which can increase our set temperature and cause a fever interleukin-1 gets produced once these immune cells are macrophages our lymphocytes actually start to destroy bacteria and then they take up one of these endotoxins they release interleukin-1 which then goes up to our hypothalamus to increase our set point thought to be through the production of prostaglandins specifically prostaglandin e2 so if you're able to inhibit the production of prostaglandin through non-steroidal anti-inflammatories then you may be able to stop that set point from get being increased and stop a fever or stopper or stop pyrexia so those are called antipyretic drugs you can also end up with a fever or pyrexia if you have some kind of tumor pressing up on the hypothalamus so then altering your set points or during surgery as well so if you do have a fever you do get the chills at the start because your body's trying to increase your body temperature to the next set point and then as you remove that pyrogen then you start to actually sweat and have a flush as you're trying to get rid of all that excess heat you've just stored now heat stroke kind of relates to hyperthermia now hyperthermia is different to pyrexia because the set point hasn't changed hypothermia heat stroke just literally means that your body temperature is too high but your set point is still normal so your body knows that your body temperature is too high it's trying to correct it back down but the systems aren't working either you're in a high heat environment or you're doing excessive work something's driving your body temperature up and the sweating the vasodilation everything that's trying to bring it back down is not working so that results in excessive loss of fluid you end up being dizzy you can vomit you can get gi signs end up with circulatory shock as all your blood volume reduces and they can actually start to kill off portions of your brain as the excess of heat becomes damaging to your organs in particular your brain so hypothermia is different to pyrexia because the set point is not altered and you just can't get rid of the excessive heat for whatever environmental reason now the last portion here is talking about the extreme cold so with the extreme cold what can happen is that your systems can start to fail and then you actually end up losing some of those regulatory responses trying to bring your body temperature back up as your cns system starts to reduce you get depression you become sleepy and then suddenly your heat control mechanisms prevent shivering and everything else so then it starts to spiral out of control now when it comes to frostbite frostbite is just when the actual surface areas of your body so your skin actually freezes so if your skin freezes that means that the water within the cells turns into ice and that just permanently damages the cells and then as soon as you thaw those ice crystals then the cell is going to be dead so you can end up with gangrene which is just ashima tissue so frostbite just instantly kills that tissue if you're unable to actually restore that blood flow and restore that heat transfer over to those areas and then that will eventually result in gangrene as it falls over and we're able to create a hypothermic environment to actually slow down metabolism without freezing the tissues and killing off the tissues just slowing down the metabolism in order to do some procedures like heart surgery so the heart doesn't need as much blood supply during open heart surgery to provide you know the food stuffs to keep it going because it has a reduced metabolism so that's the end of the chapter here i hope you enjoyed it here's a couple questions if anyone's interested what is pyrexia fever is question number one question number two is what is hyperthermia and then lastly question number three list three mechanisms of heat loss from the body if you enjoy these questions please let me know and i'll keep doing them otherwise i hope you enjoyed the 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